1. Field of the Invention
The present invention relates to an image forming apparatus for transferring toner images from image bearing members to an intermediate transfer member or to a recording medium carried by a belt member, and, more specifically, to a control operation for controlling a transfer voltage that is to be applied to transfer members when the toner images are transferred.
2. Description of the Related Art
There is an image forming apparatus for forming an image on a recording medium using image forming sections provided along an intermediate transfer member (see Japanese Patent Laid-Open No. 2002-0056587). In the image forming apparatus, in the corresponding image forming sections, toner images formed on photoconductor drums are transferred onto the intermediate transfer member at primary transfer portions by primary transfer members to which a primary transfer voltage is applied. The toner images which are primarily transferred from the image forming sections are simultaneously secondarily transferred onto a recording medium.
In contrast, Japanese Patent Laid-Open No. 05-006112 discloses a method for controlling a transfer voltage using so-called active transfer voltage control (ATVC), in which different test voltages are applied to a transfer roller to obtain a voltage-versus-current relationship, and in which a transfer voltage that causes a desired current to flow is set in accordance with the voltage-versus-current relationship.
When the control method using the ATVC is applied to the above-mentioned apparatus, a configuration is provided, in which ATVC operations are simultaneously performed on upstream and downstream image forming sections that are provided in a moving direction of an intermediate transfer belt (see Japanese Patent Laid-Open No. 11-202651). In this configuration, it is difficult to obtain the voltage-versus-current relationship resulting from stable measurement when the test voltages are applied. As a result, it is difficult to set an appropriate primary transfer voltage.
In other words, when an ATVC operation is performed, the intermediate transfer member is charged. Additionally, currents that flow when the test voltages are applied are changed in accordance with a charge state of the intermediate transfer member. When measurement of a current is performed in a downstream image forming section that is provided in a moving direction of the intermediate transfer member, it is difficult to stably measure the current because the intermediate transfer member is charged by an ATVC operation that has been performed on an upstream image forming section.
FIG. 10 illustrates a voltage-versus-current relationship obtained in a case in which one of the test voltages is applied simultaneously to all of the image forming sections. In contrast, FIG. 11 illustrates a voltage-versus-current relationship obtained in a case in which the test voltage is applied to the image forming sections at different times. Note from FIGS. 10 and 11 that the voltage-versus-current relationship obtained in a case in which the test voltage is applied simultaneously to all of the image forming sections differs from the voltage-versus-current relationship obtained in a case in which the test voltage is applied to the image forming sections at different times.
The difference between the relationships is caused by an influence of the test voltage that has been applied in an upstream image forming section. Regarding a transfer portion of the most upstream image forming section, there is no difference between the voltage-versus-current relationship shown in FIG. 10 and the voltage-versus-current relationship shown in FIG. 11. The reason for this is that the intermediate transfer member has not been charged when a current is measured in the most upstream image forming section. In contrast, when currents are measured in second to fourth image forming sections, the intermediate transfer member has already been charged by an ATVC operation performed on the most upstream image forming section. Accordingly, errors occur.
Next, an influence that charging of an intermediate transfer member has on control of a primary transfer voltage is described. FIG. 12 is a schematic diagram of a current path of a primary transfer portion. The current path can be considered to extend from a power source (not shown) connected to a primary transfer roller 5 to electrical ground. As shown in FIG. 12, the current path is divided into two paths.
The two paths are as follows: a path (1) is a path from the primary transfer roller 5 via the intermediate transfer member 7 to a photoconductor drum 1; and a path (2) is a path through which a current flows due to an influence of a capacitance of the intermediate transfer member 7. A current that is necessary for transfer of a toner image is a current that flows through the path (1). In contrast, the current that flows through the path (2) is mainly used to charge the intermediate transfer member, and it contributes little to transfer of a toner image. For this reason, in order to set an appropriate primary transfer voltage, it is necessary to accurately measure the current that flows through the path (1).
The current that flows through the path (2) can be estimated in a state in which the intermediate transfer member is not charged. Accordingly, a voltage-versus-current relationship is obtained by measuring the amounts of currents in a state in which the intermediate transfer member is not charged. Then, the amount of current that is obtained by adding the amount of current which flows through the path (2) to the amount of current according to the voltage-versus-current relationship, i.e., the amount of current that is obtained by adding the “amount of current which flows through the path (2)” to the “amount of current which flows though the path (1)”, is determined as the value of a target current. A voltage that causes the target current to flow is set as a primary transfer voltage. The primary transfer voltage is set in this manner, whereby the current that flows through the path (1) when primary transfer is performed can be easily adjusted to a desired current.
However, when the primary transfer voltage is to be set in accordance with a voltage-versus-current relationship obtained in a state in which the intermediate transfer member is charged, it is necessary to estimate the amount of charge on the intermediate transfer member.
In reality, there are a large number of related parameters, such as parameters related to a charge state of the intermediate transfer member in an upstream image forming section, and parameters related to an attenuation state after the intermediate transfer member passes through the upstream image forming section. Thus, it is difficult to estimate the amount of current that flows through the path (2).